The long term scientific goal of this proposal is to gain an understanding of the molecular mechanisms which underlie the phenotypic expression of drug resistance. Many cancer patients fail to benefit from potentially effective chemotherapy because their tumors are either intrinsically resistant or acquire resistance following exposure to sub-curative drug concentrations. Nitrogen mustards and nitrosoureas are two classes of bifunctional alkylating agents with distinct mechanisms of action. Both are useful in the clinical management of a number of malignancies, although resistant tumor cells frequently repopulate a tumor following partial response. By using nitrogen mustard resistant rat and human tumor cell lines and by analysing primary human biopsy material, the role of drug detoxification in the expression of the resistant phenotype will be assessed. Glutathione-S-transferase (GST) isoenzymes are important in xenobiotic metabolism and preliminary indications are that both quantitative and qualitative differences exist in nitrogen mustard resistant tumor cells. Because of this, a number of biochemical and molecular studies with GST are planned. These include, (a) glutathione affinity column chromatography purification and full biochemical and immunological characterization of the multiple GST isoenzymes in resistant and sensitive tumor cells. (b) Sub- cellular compartmentalization of GST isoenzymes and the relevance of such distribution to nitrogen mustard metabolism. (c) Synthesis of GST isoenzyme cDNA and subsequent evaluation of transcriptional control under conditions of selective drug exposure. (d) Drug combination studies in vitro and in vivo using pertinent GST inhibitors with nitrogen mustards in an effort to increase tumor cell kill. Such studies should establish if increased GST activity and/or the presence of novel GST isoenzymes are a consequence of specific nitrogen mustard pretreatment or are an overt cellular response to a stress environment. Concomitant to the GST analyses, O6-alkyl guanine transferase (Mer phenotype) and glutathione reductase activity will be assessed in both cell lines and biopsy material to determine the potential link between nitrogen mustard resistance and nitrosourea collateral sensitivity.